Photo meter assesses ambient light

Most PN-junction diodes can be
used as photodiodes. While not
optimized for this application, they do
work. When the diode is reverse biased,
it will produce a small photovoltaic output
as the light level is increased. LEDs
are particularly suited for this task
because their housings are transparent.

You can construct a simple circuit
that will assess the condition of ambient
lighting and, because many LEDs’ packages
are tinted to enhance their emitted
color, may even yield a reasonable evaluation
of the detected color. The results
are not as effective as those obtained
using a high-quality optical filter, which
typically has narrow bandpass characteristics,
but they can be quite acceptable.

Though the design described here
does not produce the accuracy of designs
with laboratory-grade photodetectors
and transimpedance amplifiers, it can
be quickly assembled and will produce
usable results at a low cost.

Three LEDs are used; experimentation
will indicate which device has
the best sensitivity to which color
(Figure 1). The ambient light falling
on the LEDs causes some current flow—typically in the range of 10 to 100 nA—through each LED, depending on the
applied illumination level. This current
flows through the base of a transistor,
Q1, and is amplified. Q1’s collector current
then splits between potentiometer
R4, which acts as a first-stage gain calibration,
and the base of Q2.

Q2 provides further amplification and
drives the left side of a bridge circuit
(D1A and D1B). Note that R2/D1 and
R3/D2 form a balanced bridge. Q2’s collector
current provides a slight imbalance
to the bridge. The meter, M, measures
this imbalance. R5 adjusts the sensitivity
of the meter.

Set R4 and R5 such that the meter has
an appropriate deflection. R4 is useful for
selecting the quiescent point; R5 is useful
for adjusting the sensitivity.

Before building the circuit, check
whether the LEDs can be used as
photo sensors. To determine whether
a given LED is a good photodiode,
check the voltage across the LED using
a common digital multimeter set to
its most sensitive range—typically
200 mV. Typical output voltage should
be approximately 0.3 to 1 mV with typical
office illumination.